1. Field of the Invention
The present invention is directed to a laser-based method and apparatus for the treatment of hard biological material, such as the ablation and removal of hard dental material.
2. Description of the Prior Art
Processes and devices employing a pulsed laser for removing hard biological material, such as hard dental material are described, for example, in German OS 4 030 734 and OS 3 911 871. In OS 4 030 734, a particular technique, and various types of equipment for implementing the technique, are described for treating carious teeth and for conducting root canal repairs. The devices described therein, in general, include a pulsed laser, a fiber optic transmission system, and a fiber optic laser-transmitting handpiece with interchangeable therapy heads. The source of the radiation is a pulsed alexandrite solid-state laser, operating in the wavelength range between 720 and 860 nm. By the addition of an appropriate optical module, it is possible to double the frequency, and therefore attain an operating range between 360 and 430 nm. In one embodiment of this known system, the laser beam is emitted from the removable head as a "free" beam, that is, the focus lies outside the emission plane. In another embodiment, the emission point is at the end of a light waveguide which is located in the therapy head. The laser beam is emitted as a dispersing beam in this embodiment. This embodiment is designed specifically for the preparation of root canals. The coupling into and out of light guides is generally accomplished using spherical lenses. At the point of termination, the face of both the handpiece and the therapy head contain a window made of compressed quartz glass having anti-reflective properties. This prevents dust and dirt from entering the individual components of the equipment.
In OS 39 11 871, a process is described for the removal of dental material using a pulsed infrared laser. In this known method, the dental material is covered with a thin film consisting of a fluid which absorbs laser radiation, either before or during the irradiation process. By this technique, the danger of damage to the surrounding healthy tissue is reduced, while not interfering with the efficiency of the removal of dental material. The fluid is applied intermittently, i.e., during the pauses between the respective laser pulses.
It is generally known that, using a pulsed laser system, the individual pulses of the laser radiation may exceed the threshold of critical energy concentration (which varies by material), so that biological material can be removed without creating a significantly increased temperature in the areas peripheral to the treatment location. To achieve these results, however, extremely short light pulses (on the order of nanoseconds) must be used, and the thickness of the biological material removed by this method is between 10 and 50 microns. To reach worthwhile rates of material removal, given such a tiny thickness per individual light pulse, it is necessary to increase the repetition rate of the laser pulses. Because hard biological material has a limited heat transfer capacity, however, increasing the repetition rate of the pulses rapidly leads to an accumulation of heat around the zone of removal, and hence leads to thermal damage of the areas peripheral to the treatment area. To reduce the level of thermal damage resulting from the use of laser systems wherein the laser radiation is not significantly absorbed by naturally-present water or air, various types of cooling equipment are known which generate a continuous jet of water or a continuous flow of air to the treatment site.
It is known that transmitting high intensity laser radiation through optical fibers can cause a photohydraulic phenomenon at the treatment site, which causes the biological material which has been ablated by the action of the laser pulses to impinge on the exit face of the light guide, thereby resulting in the destruction of the light guide fibers. This, in turn, requires that the treatment be terminated. In an effort to avoid this problem, the transmission of such high intensity light levels is usually accomplished by using movable arms which incorporate mirrors. Such a transmission method is disclosed, for example, in the German OS 39 11 853. This document shows an opto-mechanical endpiece for one such mirror arm for an Er:YAG laser. This arrangement permits the transmission of pulsed laser radiation,, however, such mirror systems have the disadvantage that any error in the assembly, or in the direction of the output of the laser radiation, is increased by a factor of two with each mirror present in the apparatus. Therefore, any deviations in the coaxiality of the system resulting from misalignments at the optical interface locations, or dynamic variations in the output direction of the laser radiation, or its angle of dispersion, are multiplied by two for each point at which the light changes direction. Such mirror systems are, therefore, extremely difficult to adjust. Moreover, the cost of manufacturing the mechanical parts with the necessary precision is, as a result, very high. Additionally, such mirror systems are freely movable only within limits, because considerable force is required to articulate each point of directional change, and any two points cannot always be connected with each other along the shortest path. Although it is theoretically possible to transmit optical radiation using light waveguides, the ability to transmit high intensity short laser pulses, under conditions as described above, is limited by the technical capabilities of the material currently available for the manufacture of optical fibers.